The Acorn Microcomputer was designed by Sophie Wilson, then an
undergraduate at the University of
Cambridge, England. Here, she answers some questions about those
early days. See also the timeline and people
overview.

How did the Acorn Microcomputer get started?

I had designed something similar for myself, and was in the process of
helping Hermann [Hauser] with his ideas for an ‘electronic pocket
book’ (what we might nowadays call a PDA). In the course of showing
that my designs for it would work, I showed him my schematics for my
own machine and was challenged to build it. So I did With my own
white ceramic 6502, too. That was “just” the equivalent of the CPU
board of the System 1 with LEDs and keyboard (all on the same bit of
Veroboard) – the cassette interface was added later. I think Hermann
was overly impressed when it worked first time!

No bugs in the monitor program?

No bugs in the first, smaller, version of the software (256 bytes of
code blown into PROM by Nick Toop’s PROM blower). There were a
couple of problems with the cassette interface software because I got
the order of bits the wrong way round (from the CUTS¹ standard).
But hey, it was only 512 bytes: you can’t make any mistakes in
that even when you write it by hand, hand assemble it and hand enter
it into the PROM blower! Besides, it could debug itself to a fair
extent (given that it basically worked).

Was the design based on/derived from an earlier machine? Kim-1,
Apple I, etc.?

Not exactly based on anything. Most of its heritage was from an
automated cow feeder that I’d designed for a Harrogate company
the previous summer (1977). Quite an advanced thing, really – it
had a (waterproof) number pad, big 7 segment LEDs, OS in non-volatile
EEPROM, and the trademark 6502. Both were from my own designs for
something for myself, and they came from the aether.

The most hair-raising thing was the cow-feeder’s programme. I
didn’t own a PROM blower, so I had to write the whole thing by
hand and send it off to a company who hand entered it into a machine
and sent me back the PROM. That worked first time, too. Mind you, it
was even smaller, being a boot loader that allowed the
cow-feeder’s EEPROM to be initialised.

You were an undergraduate student at Cambridge then. What were you
studying?

Maths, followed by Computer Science.

Why did you choose to design the computer around the MOS Technologies
6502 microprocessor?

Because it was there. Because it was new. Because a few of the other
members of the Cambridge University Processor Group [CUPG] were going
on about it being easier to interface to circuits. I guess I
don’t really know precisely why I chose the 6502 – maybe we
just had an affinity!

It cost quite a lot back in 1977/78 when I bought mine – which
was a wonderful white ceramic part with gold (coloured?) legs and lid.

Why did you use the RAM I/O chip, instead of a UART or something
similar, when the machine had separate RAM?

Because Hermann had them around: Science of Cambridge used 8154s on
its MK14 kit (National SC/MP based) and so they were available when we
needed something for the 6502. They were fairly cheap and the extra
RAM was a bonus, even though it meant converting from 6502 clock/write
to the read strobe/write strobe that they used.

Actually, that came later. The initial work was done for
Hermann’s own company “Cambridge Processor Unit”
(that’s an Austrian’s idea of a joke). Hermann went for the
System One and came up somehow with the Acorn name, then Clive and
Chris had an argument and Chris left Science of Cambridge and joined
Hermann at Acorn: the first thing that we worked on with Chris was the
Atom.

At the start, CPU had consultancy contracts for fruit machines.
Initially these had been SC/MP based, but they got moved to 6502s. I
was first approached by Hermann at a CUPG meeting – he wanted
someone who knew about low power technology, since he had this idea
for a “electronic notebook”. I designed an anti-theft device
for the fruit machines (piezo lighters [were] being used to knock out
electronic devices, so I put in a wideband radio receiver to stop the
fruit machine paying out mistakenly: later on the acceptance test for
the machine involved it being plugged into the same power line as an
arc welder and sparks being struck – it passed!). After that
Hermann wanted to see my designs that might work for the electronic
notebook and asked “will it work?” “Of course”
“so build it”.

When did you build the prototype?

Summer holidays, 1978. Then I went home and drew circuit boards on the
dining room table (and floor!) and wrote the manual. All by hand, of
course.

Christmas 1978 I must have written System BASIC.

When did you first show it to Hermann Hauser?

During the time it was built! Hermann was very interested in it. It
certainly worked before I went back to Yorkshire before the start of
the term.

Who designed the hardware?

Me for the bottom board. Me, Stephen Furber and maybe Kim
Spence-Jones for the top board (cassette interface). (Hmmm –
maybe KSJ was a little later – he certainly did some of the work
on the analogue bits of the BBC machine cassette interface.)

Packing them in boxes (upstairs at 4a Market Hill): the whole company
would stand around tables (a production square) and put in the right
components (me, Hermann, Hermann’s then fiancée, Stephen,
Chris). We all did pretty much anything: I ended up as Hermann’s
secretary before we could afford one!

There used to be problems with answering the phone: one chap would
ring up and say “I have got an Acorn, it does not work”
often enough for it to become a legend. We got very tired of kits
– the highlight being a guy who assembled his Atom with glue
because he knew that heat (solder) would damage them – so that
coloured the BBC machine a lot.

(The Acorn Microcomputer kit was based on the Eurocard printed circuit
board format, which meant that it was readily extended. Acorn went on
to produce a series of machines, the System 2 through the System 5,
based on the format. Acorn then used elements of the design for the
Acorn Atom in March 1980, one of the first home machines that came with
a QWERTY keyboard and a case.)

When was the last Acorn System x shipped?

We designed the BBC machine using System 3s (I did a lot of character
design work with a prototype System 80 column video card) and still
had System 4/5 stuff going on in 1982 – perhaps then.

¹
CUTS: Computer Users Tape Standard.

[Interview edited from e-mail conversations between Mike Cowlishaw and
Sophie Wilson, in January and February 2002. Thanks, Sophie.]